(a) Field of the Invention
The present invention relates to an optical disk and, more particularly, to an optical disk in which optical pre-grooves are formed for tracking control.
(b) Description of the Related Art
Recently, optical disks including a read-only optical disk, rewritable magneto-optical disk and the like have widely been used as high density recording media. Magneto-optical disks, for example, include one having a rewritable magneto-optical recording area wherein a customer stores his own data by himself and a ROM area wherein a disk manufacturer stores read-only data based on the length of a pit. In the above type of a magneto-optical disk, spiral pre-grooves (or annular pre-grooves) are generally formed for tracking control whereby tracking of a head is controlled based on a location of the pre-grooves.
FIGS. 1A and 1B are a plan view and a cross-sectional view taken along line A--A of FIG. 1A, respectively, for showing a generalized structure of an optical disk of the type as described above. The magneto-optical disk has a magneto-optical recording area 12 and a ROM area 14 on a recording surface 10 thereof. The magneto-optical recording area 12 includes magneto-optical data-tracks 16 each formed of a planar land area into which data are to be stored and optical pre-grooves 18 disposed alternately with the magneto-optical data-tracks 16 to guide an optical spot for recording/reproducing use to the centers of the magneto-optical data-tracks 16. On the other hand, the ROM area 14 includes optical data-tracks 20 wherein read-only data to be read by the optical spot are stored and optical pre-grooves 18 to guide the optical spot to the centers of the optical data-tracks 20.
Each of the optical data-tracks 20 has a structure wherein planar land portions 22 and data-pits 24 are disposed alternately with each other, whereby track numbers, sector numbers or other ROM data are recorded based on the length of the data-pits 24. In general, profiles of the land portions 16 and 22, the optical pre-grooves 18 and the data-pits 24 are formed by transcription of a stamper onto the substrate surface by employing an injection molding technology. This enabling of the transcription makes it possible to produce a great number of replicas in a low cost, which fact renders the optical disks such as a ROM disk or a partial ROM disk most valuable in delivery of identical data such as softwares in a low cost and in a vast volume. Such an ability in which data are reproduced in a vast volume and in a low cost is a most specific characteristic of the optical disks.
Detection of the optical pre-grooves 18 is performed by an optical detector 26 for a servo use divided into two sections in a radial direction of the optical disk to receive a reflected light of an optical spot travelling with a recording/reproducing head in unison. The output I.sub.1 of the first photo-sensor section 28 and the output I.sub.2 of the second photo-sensor section 30 of the optical detector 26 are arithmetically processed in an unshown signal processing circuit.
FIGS. 2A and 2B illustrate signal waveforms of outputs I.sub.1 and I.sub.2 from the ROM area and the magneto-optical area, respectively. The outputs I.sub.1 and I.sub.2 are high when the head is located at a center of the land portion while the outputs I.sub.1 and I.sub.2 are low when the head is located at a pre-groove portion. Moreover, when the head is staying in the ROM area, the outputs I.sub.1 and I.sub.2 include high frequency signal components as shown in FIG. 2A due to the data-pits 24 passing the light spot. In the ROM area, low-pass filters are used for eliminating the high frequency components in outputs I.sub.1 and I.sub.2 output from both photo-sensor sections 28 and 30 to thereby obtain an output similar to the outputs from the magneto-optical area 12 as shown in FIG. 2B.
A summed signal (I.sub.1 +I.sub.2).sub.LP given by summation of both outputs I.sub.1 and I.sub.2 after passing through the low-pass filter is applied, for instance, to counting a number of the pre-grooves that the light spot has crossed over. Similarly, a difference signal (I.sub.1 -I.sub.2).sub.LP given by subtraction of the output I.sub.2 from the output I.sub.1 after passing through the low-pass filter, is applied to detecting a track center by identification of a zero output thereof.
ISO has specified characteristics of groove signals including a cross-track signal, a cross-track minimum signal and a push-pull signal, which are obtained from optical disks and generally applied to the tracking control, in common standards for optical disk systems so that interchangeability is ensured among different drive units used for driving various optical disks such as a magneto-optical disk, a ROM disk and a partial ROM disk.
The cross-track signal is referred to as a signal proportional to a difference output given by subtraction of a first summed signal (I.sub.1 +I.sub.2).sub.LPOG, which generates from the head located in a pre-groove portion 18, from a second summed signal (I.sub.1 +I.sub.2).sub.LPOL, which generates from the head located in land portions 16 and 22, the cross-track signal being normalized by a reflected light (I.sub.1 +I.sub.2).sub.a from a substantially planar mirror surface (referred to as "normalized by the reflectance of a mirror surface" hereinafter). The cross-track minimum signal is referred to as a signal proportional to the summed signal (I.sub.1 +I.sub.2).sub.LPOG described above which generates from the head located in the pre-groove portion while the push-pull signal is referred to as a signal proportional to a difference between the outputs of both the photo-sensor sections after passing through the low-pass filters, each of the signals being normalized by the reflectance of a mirror surface.
Recently, a high densification of an optical disk has been pursued by reducing not only a pre-groove spacing (namely, a track-pitch) but also a pit spacing in a data-track to increase a recording capacity of the optical disk. A conventional method for specifying an optical disk according to ISO standards, however, cannot afford to ensure sufficient signal precision required for tracking control under the request for the high densification, especially from pre-grooves of a ROM area. To solve the problem, it has been proposed that a peak-hold circuit is to be used in a drive unit during reproduction of a pre-groove signal from the ROM area. The method will be described with reference to FIGS. 3A through 3C.
The proposed method includes passing both the outputs I.sub.1 and I.sub.2 shown in FIGS. 2A and 2B through respective peak-hold circuits to extract their upper envelopes I.sub.1PH and I.sub.2PH as shown in FIG. 8A. Thereafter, a summed signal (I.sub.1 +I.sub.2).sub.PH is given by summation of both upper envelopes as shown in FIG. 8B while a difference signal (I.sub.1 -I.sub.2).sub.PH is given by subtraction of upper envelope I.sub.2PH from upper envelope I.sub.1PH as shown in FIG. 3C. Those proposed signals (I.sub.1 +I.sub.2).sub.PH and (I.sub.1 -I.sub.2).sub.PH can be utilized for head tracking similarly to the conventional summed signal (I.sub.1 +I.sub.2).sub.LH and difference signal (I.sub.1 -I.sub.2).sub.LH.
The proposed summed signal (I.sub.1 +I.sub.2).sub.PH has an amplitude larger than the amplitude of the conventional summed signal (I.sub.1 +I.sub.2).sub.LH, so that an error rate can be suppressed in counting a number of tracks that the head has crossed over by identification of a maximum or minimum value thereof. The proposed difference signal (I.sub.1 -I.sub.2).sub.PH given by the method has both an excellent linearity and a steep slope at a zero signal as compared with those of the conventional difference signal (I.sub.1 -I.sub.2).sub.LP, so that an advantage of a higher precision is obtained in tracking of a head toward a track center.
However, a structure of a high density optical disk having a track-pitch less than about 1.2 .mu.m and suitable for the driving units which employ the proposed method described above has not been known.